This invention relates to a two-dimensional detector of ionizing radiation and a process for manufacturing this detector.
For example, the invention can be used to detect X photons, gamma photons, protons, neutrons and muons.
The invention is particularly applicable to the following domains:
experiments in detonics,
fast non-destructive testing,
position of patients in radiotherapy,
high-energy physics,
neutronography, protonography, radiography, gammagraphy,
surgery under radioscopy, and
safety in airports
Two-dimensional detectors of ionizing radiation made of plates of a heavy meal such as lead or more generally a material capable of interaction with an incident ionizing radiation, are already known.
In particular, US S 117 114 A describes a radiation detector. In one example, the detector comprises a sequence of plane and parallel detection matrices. Each matrix comprises a set of amorphous silicon detection cells equipped with addressing electrodes and an adjacent layer made of a metallic conversion material such as Pb or U. This makes it possible to determine the energy and initial position of an X photon or a gamma photon that arrives at the first layer of metallic conversion material, perpendicular to this layer.
U.S. Pat. No. 4,210,805 A describes a radiation detector. In one example, eight semiconductor elements 30 are laid out in the form of a matrix. Two opposite faces on each element are covered with conducting layers and the other two faces are covered with insulating layers that separate them from neigbouring elements.
For example, it is known how to use a metal with atomic number Z equal to or greater than 73 to detect X or gamma photons and a material with an atomic number Z usually less than 14 or greater than 90 to detect neutrons. Other materials such as gadolinium (Z=64) can also be used to detect neutrons.
The plates are perforated with holes by chemical or electrochemical etching and are electrically insulated from each other if necessary (when the required thickness of the plates is equal to 100 or more micrometers).
The holes are filled with an ionizable gas.
An incident high energy X or gamma photon then generates at least one photoelectron in one of the plates of the detector, by the Compton effect or by the pair creation effect.
This incident X or gamma photon communicates fast movements to this electron with a kinetic energy of the order of magnitude of the energy of the incident photon. This fast electron then ionizes some gas molecules contained in one of the holes into which the electron arrives, and through which the electron usually passes.
The slow secondary electrons that are torn off these molecules due to ionization of these molecules, are guided along this hole and collected using an electrical bias field, also called an electrical drift field, and are then detected, for example, in an ionization chamber or in a proportional avalanche chamber.
This type of two-dimensional detector is described in a number of documents including references [1], [2], [3], [4] and [5] mentioned at the end of this description.
A detection structure with holes is chosen because this type of structure is known to be very conducive to obtaining a good spatial resolution and a good efficiency, provided that the holes are perfectly formed and are sufficiently large.
These holes are formed by chemical etching. This method is preferred to water jet cutting which generates a front shock when the jet is opened, when starting to perforate a hole.
This front shock causes scaling of the material in which the holes are to be formed, which causes spalling of this material and makes it unsuitable for use.
But chemical etching is a slow and expensive technique.
Furthermore, the efficiency at which secondary electrons are collected and therefore the efficiency these hole detectors are limited because this technique is used. For example, only 10 to 30% of the secondary electrons created during each gas ionization are typically collected.
Chemical etching cannot be used to produce holes with sufficiently cylindrical walls because it generates narrow points in the holes that deform the electric field lines and reduce the useful diameter of the holes, with the result that the global efficiency of hole detectors is limited.
The purpose of this invention is to overcome these disadvantages of high cost and limited efficiency.
More precisely, the purpose of this invention is a two-dimensional detector of incident ionizing radiation composed of first particles. This detector comprises a stack of sheets of a first material capable of emitting second particles by interaction with the incident ionizing radiation. This detector is characterized in that it also comprises:
layers of a semiconducting material that alternate with sheets of the first material and may be ionized by the second particles, each of the layers being associated with one of the sheets, the stack having first and second opposite faces each containing corresponding edges of sheets and layers, the detector being designed to be laid out such that the ionizing radiation arrives on the first face, the length of each sheet measured from the first face as far as the second face being equal to at least one tenth of the free average path to the first particles in the first material,
groups of parallel and electrically conducting tracks extending from the first to the second face parallel to the layers, each group being associated with one of the layers and in contact with it, the tracks being designed to collect charge carriers that are generated in the layers by interaction of the layers with the second particles and possibly with the first particles and that are representative of the first particles in intensity and in position, and
means of creating an electric field capable of causing collection of charge carriers through the tracks.
The detector according to the invention can be made at a much lower cost than the hole detectors mentioned above.
Furthermore, the collection efficiency and the spatial resolution of the detector according to the invention may be very much greater than the corresponding values for hole detectors.
According to a particular embodiment of the detector according to the invention, the first material is electrically conducting, the tracks are electrically insulated from the sheets and the means of creating the electric field comprise means of applying a voltage between the tracks and the sheets, this voltage being sufficient to cause the collection of charge carriers through the tracks.
Preferably, each group of tracks is fully located within the layer with which it is associated.
In this case, according to another particular embodiment, the first material is electrically conducting and the means of creating the electric field comprise means of applying a voltage between the tracks and the sheets, this voltage being sufficient to cause collection of charge carriers through the tracks.
According to another particular embodiment, the sheets are electrically insulating, an electrically conducting layer is inserted between each layer of semiconducting material and the sheet that is associated with it and the means of creating the electric field comprise means of applying a voltage between the tracks and the electrically conducting layers, this voltage being able to cause collection of charge carriers through the tracks.
The semiconducting material may be crystalline, ceramic, vitreous, amorphous or polymer.
It may be chosen among the group including thin layers of diamond, CdTe, ZnTe, CdZnTe, AsGa and particularly AsGaAlxPlxe2x88x92x (0 less than x less than 1), InP, InSb, SiC, crystalline silicon, amorphous silicon, organic crystals for example such as anthracene, naphthalene and PPV, amorphous selenium and chalcogenic glass (As2S3).
The detector according to the invention may also comprise an electronic device for reading electrical signals output by tracks when the tracks collect charge carriers.
According to one preferred embodiment of the invention, one end of each track is curved to extend onto an edge of the corresponding layer of semiconducting material, this edge being located on the second face of the stack, and the device comprises electrically conducting pads that are in contact with the curved ends of the tracks respectively.
This invention also relates to a process for manufacturing the detector according to the invention.
According to this process, a layer of semiconducting material is formed on each sheet, where this layer is provided with the group of tracks associated with it, and the sheets are provided with layers of semiconducting material. The tracks are assembled together to obtain a stack in which these layers of semiconducting material alternate with the sheets.
According to one particular embodiment of the process according to the invention, a first layer of semiconducting material is formed on each sheet, where the thickness is less than the thickness of the layer of semiconducting material. The group of tracks is formed on this first layer and a second layer of semiconducting material that covers these tracks is formed on the first layer. The total thickness of the first and second layers is equal to the thickness of the layer of semiconducting material.
It would also be possible to deposit a half layer of semiconducting material on the two opposite faces of two successive sheets, and then form the group of tracks on one of the half layers and assemble the sheets thus covered to create a stack in which the layers alternate with the sheets.